1. Field of Invention
This invention relates generally to optical communications and in particular to arrayed waveguide grating (AWG) devices.
2. Relevant Technology
Arrayed Waveguide Grating (AWG) devices have been commercially successful in the optical communications industry. Tens of thousands of AWGs, made with Silica-on-Silicon planar waveguide integrated optics technology, have been deployed in today's communication networks. Their main application is in dense wavelength division multiplexing (DWDM) systems where multiple communication channels (wavelengths) are combined (multiplexed) and transmitted on a single fiber over large distances and separated (de-multiplexed) at the receiving end to recover the individual signals. The process of multiplexing and de-multiplexing wavelengths is done via the AWGs. Although other technologies exist for performing the same operation as the AWGs, AWGs are the most prevalent due to their superior performance, scalability and reliability.
FIG. 1 is a schematic illustration of a conventional multiplexer (MUX) device 50 having an AWG unit 40. An AWG unit, which is currently available in the form of a monolithic chip, is an integrated optics planar waveguide structure that acts like a bulk diffraction grating. The construction and operation of AWG units are well known in the art. The AWG unit 40 has an input free propagation region (FPR) 42 and an output FPR 44 connected by an arrayed waveguide grating region 46. The arrayed waveguide grating region 46 includes channel waveguides of varying lengths. The waveguides usually vary in length by increments of ΔL such that, if there were seven waveguides, their lengths would be χ+3ΔL, χ+2ΔL, χ+ΔL, χ, χ−ΔL, χ−2ΔL, and χ−3ΔL. Different wavelengths traveling through the array experience different amounts of time delay. The interference and diffraction caused by the different amounts of delay in each waveguide causes the radiation components having different wavelengths to emerge at different angles from the output end of the array waveguide grating region 46.
As shown, separate signal wavelengths (shown as λ1, λ2, λ3, λ4 and λ5 in FIG. 1) enter the AWG device 40 through multiple input waveguides, pass through the arrayed waveguide grating region 46, and exit the multiplexer device 50 as a multiplexed signal. The input waveguides connect with the input FPR 42 at input face F1. The passband shape for a typical MUX device is wide-band Gaussian.
Although AWGs can perform both multiplexing and demultiplexing operations, the requirements for the two operations are usually quite different. More specifically, the demultiplexing operation has narrow spectral passband width requirements and tight adjacent channel crosstalk requirements, while the multiplexing operation has wide passband width requirements and no crosstalk requirement. The wavelength channels that are combined or separated usually conform to the ITU (International Telecommunications Union) grid—i.e., their frequencies are separated by a fixed spacing. For example, the wavelength channel frequencies may be 50 GHz, 100 GHz, and so on.
FIG. 2 shows the passband shape and the peak transmission of a waveguide channel. Typically, the spectral passband is defined at 0.5 dB, 1.0 dB and 3.0 dB below the peak transmission value. FIG. 2 shows the “1-dB passband width” and the “3-dB passband width,” which are the widths of the passband (in nm) at 1 dB and 3 dB below the peak transmission, respectively.
Table 1 below compares the passband widths (in nanometers) for different AWGs that are designed to handle both multiplexing and demultiplexing operations. These passband widths correspond to an AWG designed for a channel separation of 50 GHz (or roughly 0.4 nm).
TABLE 1Comparison of spectral passband widths for different applications0.5 dB1.0 dB3.0 dBPassbandPassbandPassbandPassbandShapeAWG FunctionWidth (nm)Width (nm)Width (nm)GaussianDemultiplexer0.050.10.2Flat-topDemultiplexer0.150.20.3GaussianMultiplexer0.0750.150.3Flat-topMultiplexer0.150.20.3Ultra wideMultiplexer0.40.50.6Flat-top
As shown in Table 1, there are different passband shapes and a range of passband widths that can be achieved. As DWDM systems move toward narrower channel spacing and higher modulation rate, the passband of multiplexer filters needs to be much wider than conventional flat-top filters. Hence, the ultra wide flat-top passband is becoming more desirable. FIGS. 3A and 3B show the spectral passband shapes of a flat top demultiplexing channel (second row in Table 1) and an ultra-wideband flat-top multiplexing channel (last row in Table 1). Although both passbands are flat-top spectral passbands, the former is narrower and has better crosstalk (i.e., sharper edges) than the latter. The passband shape of the ultra-wideband flat-top multiplexing channel is wide enough such that even the 0.5-dB down passband width is as wide as the channel spacing (here 0.4 nm).
In addition to the passband width, another characteristic of interest is the ripple, which is pictorially depicted in FIG. 4. “Ripple” refers to the flatness at the top of a spectral passband, and there is usually a limit set on the acceptable depth of a ripple (e.g., less than 3 dB).
A method for designing an ultra-wideband, low ripple, flat-top, multiplexing AWG device is desired to be able to accommodate system demands for narrower channel spacing and higher modulation rate.